Clouded vitreous enamel



' Patented Dee-21, 1937 PATENT. OFFICE CLOUDED VITREOUS ENAMEL IgnazKreidl, Vienna, Austria No Drawing. Application February 9, 1933,

I Serial No. 656,047

13 Claims.

This application is a substitution for and continuation in part ofapplicant's co-pending ap plications, Serial Nos. 270,152, filed April14, 1928, and Serial No. 270,153, filed April 14, 1928; Serial No.371,043, filed June 14, 1929, and Serial No. 656,048, filed February 9,1933.

The art of producing clouded enamels in the enameling of iron ware orthe like is well known and has been carried out, up to the present, in-

what is known as the wet enameling process in the following manner;

Firstly, an enamel frit composition is prepared by melting together theraw enamel mixture, i. e., a. mixture of glass forming constituents,which frit, after cooling, is ground on a mill with water in thepresence of clay, and a white clouding agent, such as are generally usedfor example tin oxide, zirconium oxide, antimony oxide or antimoniates;the quantity of the white clouding agent added amounting from about 6 to8% of the by the clay itself is negligible or at least commerciallyinsuflicient due to the small opaquing effect of the clay which is borneout by the fact that in addition to .the clay a white clouding agentalways has to be added. l

, The ground frit, together with the water, the white clouding agent andthe clay, constitutes the "slip with which the wares to'be enameled arecoated, then dried at temperatures ranging from room temperature to 80?C., the drying be-' ing carried out for periods varying according .tothe temperature employed from a quarter of an hour to 12 hours, afterwhich the enamel is burned or fired at a temperature of about 750-850'C. for periods varying according to the size of the article to beenameled from 2 to 5 minutes.

.-The clouding effect in these known processes is rlue to the fact thatthere exist" in the enamel in a finely divided state solid substanceswith a refraction index much higher than that of glass, which substancesin burning the enamel remain undissolved therein, thereby efl'ecting awhite coloring. Y 3

As enamel frit, in the known processes already formed glasses have beenused; which were preclouded by adding raw substances containing lfluorine to the raw glass mixture. But such adwhite insoluble cloudingagents, by using, in lieu of some or all of the clouding oxides,substances which evolve gases. and vapors in the enamel during theburning thereof, forming minute gas bubbles which are retained in anduniformly distributed throughout the fired enamel mass and causerefraction or diffraction of light clouding medium that the cloudingefiect is brought about entirely by the presence of an insolubleclouding agent itself, being finely distributed in the enamel, but onthe contrary either in a substantial part or completely by therefraction or difiraction of lightproduced by bubbles formed by thegases evolved and retained by the enamel the actual quantities of suchgases amounting to only fractions of the quantities of the substanceswhich had formerly to be used for clouding. This will be readilyunderstood, since for a gas clouding only the volume of the evolved andretained gas counts, the weight of which is very small. While in theknown processes the solid clouding media have to be added to the enamelmass in quantitiesfrom 5 t'o10% of the weight of the enamel batch,thegas evolving substances are employed only in such quantities which leavein theenamel a certain gas volume necessary for clouding the weight ofwhich gas amounts, for instance for CO2, to only 0.05 to 0.06% of theweight of the enamel frit.

Therefore, even the color of the residue if any, of the gas cloudingsubstance plays no part, and, hence, even colored and impure materialscan be employed.

Now, according to one phase of the invention, the'gas clouding isaugmented by using the gas clouding means in conjunction with solidclouding agents, in case a raising of the gas clouding should bedesired. Thus, according to this feature of the invention, the resultingclouding in the enamel will be due only in part to the fine distributionof the white insoluble oxides in the enamel mass and in part to thepresence of gas bubbles finely distributed throughout the enamel masswhich gas bubbles act by refraction or diffraction of light. Thereby, asubstantial portion of the expensive solid whiteclouding agentsheretofore exclusively used can be saved without impairing the cloudingefiect in the least. This will be appreciated from the fact that while,according to conventional practice, the white insoluble. clouding agentsare added to the enamel batch or hit on the mill in quantities of from 6to 8 per cent of the weight of the enamel frit, according to the presentinvention the quantity of these insoluble oxides can be reduced tofractions of their former amounts, and the deficiency in the cloudingwhich would ordinarily thereby result is made up by the addition to theenamel batch of exceedingly small quantities of relatively inexpensivegas-evolving volatile or decomposable substances which may be eitherorganic, inorganic or organic-inorganic compounds.

The employment of such mixtures or compounds of solid white cloudingagents with substances which are solely eii'ective by virtue of theirgas-evolving property at the burning of the enamel, enables the whiteinsoluble clouding agent to be favorably dosed, since for a givenquantity of the insoluble clouding agent, the most favorable amount ofthe gas clouding agent to be added thereto, can be easily ascertained bytrial, thereby rendering the clouding eifect' of the enamel quite easilycontrollable.

The present invention also possesses the advantage of enabling whiteclouding agents which have only insufllcient covering power for use inpreparing satisfactory cloudings, to be used in conjunction withgas-evolving means. In other words, the present invention renders itpossible to use inferior qualities of tin oxide, zirconium oxide and thelike, as white clouding agents, which hitherto could not be used forthese purposes because of their poor physical properties, it being knownthat not all oxides of tin, zirconium and the like are satisfactory foreffecting cloudings.

In carryingout the present invention the procedure to be followed is thesame as is usual in the known processes for enameling iron warehereinbefore set forth and diflers therefrom only in that in lieu of apart or all of the known white insoluble oxides heretofore added to theenamel mass on the mill as clouding media, substances are added whichper se are incapable of serving as clouding agents, but which, onburning of the enamel, evolve gases which provide the clouding.

The invention is applicable to enamel-batches of well known compositionnow universally used, and all that is necessary-to do is to add on themill inlleu of the known white insoluble oxides heretofore used asclouding medium, a gasevolving substance, and determine empirically theamount of said gas-evolving substances required for the best results.

Such gas cloudings can be produced both with materialswhich arecompletely volatile in the process of burning the enamel or which passby decomposition completely into gaseous form, and

also with materials which are only partly volatile during the burningprocess or which only partially pass by decomposition-into gaseous form.

When substances are employed which are only partially volatile or whichonly partially pass into gaseous form during the process of burning theenamel, the characteristic of the invention of the present process isonly brought about by that portion which is volatile and passes intogaseous form in the enamel upon the burning thereof and is retained inthe enamel. Also such clouding media, therefore, which are onlypartially volatile or which only partially pass over into the gaseousform in the" molten mass upon the burning of the enamel and whichpossess a large volatile portion, will function as clouding agents whenused in quantities which are much less than the amount of clouding mediahitherto usual, and, in fact, will function when used in fractions ofthese. quantities.

That the volatile matter is responsible for the increase in cloudingover that which could be secured by the frit used and-clay alone, orover that secured by the frit used and clay with an insuiiicientproportion of insoluble oxide, is proven by the fact that if the gasevolving substance is omitted, the satisfactory increase in clouding isnot secured.

It will thus be apparent that the present invention discloses anentirely new group of clouding media.

In the process according to the present invention also, even substancesthat are soluble in water can be employed as clouding media, pro videdthey are volatile or comprise a volatile portion which will be liberatedduring the burning of the enamel.

An essential requirement in practicing thepresent invention is that,upon the burning of the enamel, only such quantities of gas are presentor evolved that no seriously harmful escape of the bubbles of gasthrough the surface of the enamel coating after firing to smoothness canoccur, and that the quantity of gas present or evolved and retained inthe enamel is sufllciently great to bring about the cloudings byrefraction or diffraction caused by the finely distributed gas bubblesin the enamel.

Accordingly, if in practicing the present invention it is found, uponthe opaquing of the enamel,

that an outwardly visible froth occurs, which would cause the surface ofthe enamel to become pierced, blistered and lustreless, the addition ofthe clouding medium must be diminished or the viscosity of the batch ofenamel so varied that this phenomenon does not occur.

The viscosity can be'diminished by adding the known fluxing agents suchas fluor-spar, borax or the like, and it can be increased by addingclay, kaolin, quartz or the like.

I have furtherdiscovered that discolorations which may occur in theburning of the enamel, for example, by the deposition of carbon or bythe decomposition of carbon compounds that fade or change color can beobviated by adding to the clouding medium or to the batch of the enamelon the mill oxidizing agents, such as saltpetre, chlorates and the like,which promote the decomposition of such carbon containing cloudingmedium.

I have further discovered that the formation of gas can also be broughtabout or promoted indirectly by the reaction of two or more substances,for instance, by a carbon compound with an oxidizing agent that causesor promotes its combustion during the burning of the enamel.

As clouding media there are available both the purely organicsubstances, compounds and .com-

plexes, and also the inorganic or inorganicorganic substances, compoundsand complexes in which a non-volatile portion eventually present, nomatter whether soluble in the enamel flux, or not, does not act as aknown clouding agent. The availability of all these substances is, of.course, based on the assumption that they can pass wholly or partly intogaseous form during the burning of the enamel.

To name only a few examples of organic clouding media which fall withinthe scope of the present invention there may be mentioned organic acids,both of the aliphatic and of the aromatic series, and their salts,alcohols, amides, carbohydrates and all substances which fulfill theaforementioned conditions. 'The number of these substances is so greatthat it would be superfluous to enumerate" them. As specific examples oforganic clouding media may be mentioned the fatty acids and their saltssuch as formic acid and its salts, acetic acid and its salts,

oxalic acid and its salts, benzoic acid and its salts, naphthol,benzidine, starch, flour, and the like.

Examples of inorganic substances are salts of nitric acid, salts ofnitrous acid, bicarbonates or mixtures of these substances. As specificexamples may be mentioned the bicarbonates of alkali metals, nitrites ofalkali or earth-alkali metals, nitrates etc.

The quantity of clouding medium requisite ior the clouding admits ofbeing ascertained empirically for each material without difficulty. Itis dependent upon the volatile portion in the molten mass or the portionthat passes into gaseous form during the process of burning the enamel.It is also dependent upon the permeability of the enamel batch withregard to the bubbles of gas formed therein.

The clouding is judged by comparing the-cloud- 'ing with a sample platewhich has been clouded in the usual manner with tin oxide, usually about6% tin oxide; the essential point always being whether the clouding iscapable of replacing the customary clouding which is obtained with tinoxide.

Examples (I) The following are examples of enamel mixtures to befritted. Grams (1) Borax About 19.0 Felspar About 35.5 Quartz About 16.0Sodium silico-fluoride About 17.8 Kaolin A About 2.8 Soda About 4.4Saltpetre About 3.1

(2) Borax About 21.4 Felspar About 37.0 Quartz About 11.4 Sodiumsilico-fluoride About 17.8 Kaolin About 5.7 Soda About 4.3 Sodiumnitrate About 2.1

(3) Borax About 24.6 Felspar About 24.6 Quartz About 25.7 Sodiumsilico-fiuoride About 14.0 Kaolin About 2.1 Cale spar--. About 1.6 SodaAbout 3.2 Sodium nitrate About 4.2 (4) Borax About 22.0 Felspar About30.4 Fluorspar About 1.6 Soda -1 About 8.3 .'.2uartz About 18.2Potassium nitrate About 2.5 Kaolin About 3.0 Kryolithe About 14.0 (5)Felspar About 35.0 Quartz About 5.7 Sodium silico-fluoride About 12.6Soda About 6.8 Potassium nitrate About 5.0 Borax About 20.6 Kaolin About12.6 Magnesium oxide About 0.7

(6) Borax; About 88.0 Soda -1", About 30.0 Kryolithe About 36.0Fluorspar About 16.0 Potassium nitrate About 8.0 Quartz About. 60.0

. Felspar' About 160.0

(11) Examples of clouding agents for the production of white cloudedenamels from the above "mentioned frlts:

(a) 1000 grams of any one of the above men- 'tioned frits are ground inthe ordinary way afterwards'burned at about 800 C.

b) 1000 grams of any one of the above mentioned frits are ground in theordinary way with 400 to 450 grams of water, 0.2 gram fl-naphthol asclouding agent and 60 grams clay, then applied to the objects tobeenameled and further worked in the same manner as in Example (0).

(c) 1000 grams of any one of the above mentioned frits are ground in theordinary way, with 400. to 450 grams of water, 0.3 gram benzoic acid asclouding agent and 60 grams clay, then applied to the objects to beenameled and further worked in the same manner as in Example (a).

(d) 1000 grams of any one of the above mentioned frits are ground in theordinary way with 400 to 450 grams of water, 14 grams formic acid asclouding agent and 60 grams of clay, then applied to the objects to beenameled and further worked in the same manner as in Example (a).

(e) 1000- grams of any one of the above mentioned frits are ground inthe ordinary way with 400 to 450 grams of water. 12 grams crystallineoxalic acid as clouding agent and 60 grams clay, then applied to theobjects to be enameled and nfurtherworked in-the same manner as inExample (a).

(f) .1000 grams of any one of the above mentioned frits are ground-inthe ordinary waywith 400 to .450 grams 01 water, 1 gram potassiumnitrate as clouding agent and 100 grams clay, then applied to theobjects to be enameled and further worked'in the same manner asinExample (a).

r (g) 1000 grams of any oneof the above mentioned irits are ground inthe ordinary way with 400 to 450 grams of water, 3 grams sodium acetateas clouding agent and 60 grams clay, then applied to the objects to beenameled and further worked in the same manner as in Example (a).

(h) 1000 grams of any one of the above mentioned frits are ground in theordinary way with plied to the objects to be enameled and further workedin the same manner as in Example (a).

(i) 1000 grams of any one of the above mentioned frits are ground in theordinary way with 400 to 450 grams of water, 2 grams of starch asclouding agent 0.5 gram saltpetre and grams clay, then applied to theobjects to be enameled and further worked in the same manner as inExample ((1).

(7c) 1000 grams of anyone ofthe above mentioned frits are ground in theordinary way with 400 to 450 grams. of water, 0.2gram benzldine asclouding agent, 0.5 gram saltpetre and 60 grams clay, then applied tothe objects to be enameled and further worked in the same manner as inExample (a). i

In Examples (i) and (k) the clouding agent (starch and benzidine) isemployed together with an oxidizing agent (saltpetre) which promotes thedecomposition of the clouding agent during burning the enamel. i

(l) 1000 grams of anyone of the above men- 400 to 450 grams of water, 20grams tin oxide and 20 grams ammonium oxalateas clouding agent and 60grams clay, then applied to the objects to be en'- ameled and driedthereon at temperatures of 60 to 100 C. and afterwards burned at about800 C.

(m) 1000 grams of any one of the above mentioned frits are ground in theordinary way with 400 to 450 grams of water, 15 grams zirconium oxideand 0.2 gram ,s-naphthol as clouding agent and 60 grams clay, thenapplied to the objects to be enameled and further worked in the samemanner'as in Example (a) (n) 1000 grams of. any one of the abovementioned frits are ground in the ordinary way with 400 to 450 grams ofwater, 20 grams zirconium oxide and 0.3 gram benzoic acid as cloudingagent and 60 grams clay, then applied to the objects to be enameled andfurther worked in the same manner as in Example (11).

(o). 1000 grams of any one of the above mentioned frits are ground inthe ordinary vrray with 400 to 450 grams of water, 10 grams zirconiumoxide and 0.2 gram benzidine as clouding agent and 60 grams of clay,then applied to the objects to be enameled and further worked in thesame manner as in Example (a).

What I claim is:'

1. An article of manufacture comprising a metal article having thereon acoating of clouded vitreous enamel produced from a slip containing clayand having 'an opacity substantially greater than that which would beproduced with the frit used and clay alone, a substantial part of theincrease in opacity over that caused by the frit and clay alone beingcontributed by gas bubbles which are retained in the fired enamel.

2. An article of manufacture comprising a metal article having thereon acoating of clouded vitreous enamel produced from a slip containing clayand having an opacity substantially greater than that which would beproduced with the fritused and clay alone, a major part of the increasein opacity over that caused by the frit and clay alone beingcontributed-by gas bubbles which are retained in the fired enamel.

3. An article of manufacture comprising a metal article having thereon acoating oi clouded vitreous enamel .produced from a slip, containingclay and opaquing metal oxide and having an opacity substantiallygreater than that which would be produced with the trit used, the oxideand clay alone, the increase in opacity over that caused by the hit,oxide and clay alone being contributed by gas bubbles which are retainedin the fired enamel.

4. An article of manufacture comprising a metal article having thereon acoating of clouded vitreous enamel produced from a slip containing clayand having an opacity substantially greater than that which would beproduced withthe frit used and clay alone, substantially all of theincrease in opacity over that caused by the irit and clay alone beingcontributed by gas bubbles.

which are retained in the fired enamel.

5. In a method of producing a metallic article retained in the enamel atthe firing temperature thereof in the form of bubbles of such sizes andnumber as to contribute a substantial increase to the opacity.

6. In a method of producing a metallic article having thereon a. coatingof clouded vitreous enamel having 'an opacity substantially greater thanthat which could be producedg-with the int used and clay alone, and inwhich a major part of the increased opacity is contributed by gasbubbles retained inthe enamel, the step which comprises incorporating inthe enamel slip containing clay a gas-evolving-agent which, upon thefiring oi the enamel, evolves gases which are retained in the enamel atthe firing temperature thereof in the form of bubbles of such sizes andnumber as to contribute a substantial increase to the opacity.

7. In a method oi producing a metallic article having thereon a coatingof clouded vitreous enamel having an opacity substantially greater thanthat which could be produced with the frit used and clay alone, and inwhich substantially all of the increased opacity is contributed by gasbubbles retained in the enamel, the step which comprises-incorporatinginthe enamel slip containing clay a gas-evolving agent which, upon thefiring of the enamel, evolves gases which are retained in the enamel atthe firing temperature thereof in the tom of bubbles of such sizes andnumber as to contribute a substantial increase to the opacity.

8. The method of producing a metal article having thereon a coating ofclouded vitreous enamel having an opacity substantially greater thanthat which could be produced with the frit used and clay alone, and inwhich a substantial part of the increased opacity is contributed by gasbubbles retained in the enamel, which comprises applying to the articlea slip comprising an enamel i'rit, clay and a gas-evolving agent which,upon the firing of the enamel, evolves gases which are retained in theenamel at the firing temperature thereof in the form of bubbles of suchsizes and number as to contribute a substantial increase to the opacity,and firing the slip on the article.

9. A method according to claim 5, in which the gas evolving agentcomprises an organic compound.

10. A method according to claim 5, in which there is incorporated in theslip a substance which causes or promotes the' decomposition of thegas-evolving agent into gas.

11. A method according to claim 5, in which there is incorporated in theslip an oxidizing agent.

12. A method according to claim 5. in which there is incorporated in theslip an oxidizing agent selected from the group consisting oi nitratesand chlorates.

18. An enamel slip for clouded enamels for metal were to produce thereona coating of clouded vitreous enamel having an opacity substantiallygreater than that which could be produced with the irlt used and clayalone, and in which a substantial part 01' the increased opacity iscontributed by gas bubbles retained imthe enamel, comprising a groundmixtu e of enamel frit, clay, water and a gas-evolving agent which,upon' the firing of the enamel. evolves gases which are retained in theenamel at the firing temperature thereof in the form of bubbles of suchsizes and number as to contribute a substantial increase to the opac ty.

IGNAZ KREIDL.

